Add gnu tar and zstd as pre-requisites for windows self-hosted runners

[lvpx]

Description

Update README to include gnutar and zstd in Pre-requisites section.

Motivation and Context

There have been customer requests to document installation of gnu tar and zstd for self-hosted runners. Most recent request [here].(#984 (comment)) We had them documented in workarounds before. Adding them to pre-requisites for better visibility for folks using self-hosted windows runners.

How Has This Been Tested?

N/A

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (add or update README or docs)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.